Organic light emitting diodes (OLEDS) are an emerging display technology. In essence an OLED (or organic electroluminescent device) comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied, light is emitted. At least one of the electrodes must be transparent to visible light.
There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. However, several manufacturing issues still have to be resolved before solution processing of OLEDs can fulfil its potential. In a multi-colour or full-colour display the emissive organic layers need to be patterned according to the pixel layout. High-resolution displays require a high-resolution pattern for the emissive layer. To date, solution-processing techniques for patterning the emissive layer are far from ideal.
Recent advances in OLED efficiencies have been made using phosphorescent rather than fluorescent emitters. Simple spin statistics would predict that one singlet exciton is formed for every three triplet excitons. In most organic compounds only the singlet states are emissive, so the maximum potential internal efficiency is 25%. (Note, however, that the simple spin statistics may not be applicable to conjugated polymers.) However, in phosphorescent compounds, the triplet states are emissive, which allows a greater proportion of the excitons to be utilised. Phosphorescent compounds typically have strong spin-orbit coupling, for example due to the presence of a heavy element, such as Ir or Pt. In many cases, the most efficient OLED devices have multi-layer structures (e.g. WO 00/57676 and U.S. Pat. No. 6,303,238). Such multi-layer structures can be formed by thermal evaporation, but when solution-processing techniques are used, depositing a second layer may wash away the first layer.
In an organic light-emitting display the organic light-emitting layer is generally divided into individual pixels, which can be switched between emitting and non-emitting states by altering the current flow through them. In general, the pixels are arranged in orthogonal rows and columns, and two arrangements for addressing the pixels are in common use: passive matrix and active matrix. In a passive-matrix device, one of the electrodes is patterned in rows and the other in columns, and each pixel can be made to emit light by applying an appropriate voltage between the row and column electrodes that intersect at the pixel in question. Each row is addressed in turn for a fraction of the frame time. There is a practical limit on the number of rows that can be driven with a passive-matrix addressing scheme. In an active-matrix display, circuitry (typically a transistor or combination of transistors) is provided for each pixel so that each pixel can be left in an emitting state while another pixel is addressed. Integration of the active-matrix circuitry and the organic light-emitting device is more complicated than for a passive matrix display but active-matrix OLEDs are expected to have benefits in terms of size of display, power consumption and lifetime.
It has been recognised that if a photolithographic technique could be successfully applied to the patterning of the organic layers in an OLED then this would offer many benefits. Photolithographic techniques are established in other industries and can give good resolution and high throughput. However the attempts to use photolithography during the formation of the organic layers in OLEDs have all had only very limited success.
BASF (U.S. Pat. No. 5,518,824) discusses the principle of forming an OLED using a crosslinkable charge-transporting material. The material is deposited from solution, and then exposed to UV light, which crosslinks the material making it insoluble. Subsequent luminescent or electron transporting layers can be deposited on top of the insoluble layer. BASF mentions that if the UV exposure is carried out through a mask, then the exposed areas will be insoluble and the unexposed areas still soluble, and developing (washing) this film in solvent will remove the unexposed material, leaving the insoluble patterned material. However, this patterning is not demonstrated. BASF discuss doping the film with a fluorescent dye or using a crosslinkable fluorescent dye (U.S. Pat. No. 5,922,481) to form the light-emitting layer. The EL device results reported by BASF from its crosslinked devices are very poor. The two devices reported, which have cross-linked but un-patterned light emitting layers, give light emission only at 87 V and 91 V, respectively, both of which are entirely unacceptable operating voltages for an OLED.
Bayerl et al. (Macromolecules 1999, 20, 224-228) used crosslinked oxetane-bisfunctionalized N,N,N′,N′-tetraphenyl-benzidine as the hole-transporting material in a two-layer device, in which the electron-transporting layer consisted of a poly(α-methylstyrene) matrix doped with 2-biphenyl-5-(4-tert-butylphenyl)-3,4-oxadiazol (50 wt %) and the emitter perylene (1 wt %). This device gave blue emission. However, they did not pattern the hole-transporting material. Bacher et al. (Macromolecules 1999, 32, 4551-4557) demonstrated photo-crosslinking of a hole-transporting material. They produced a patterned photo-crosslinked hole-transport layer on to which they deposited an emissive layer (tris(8-hydroxyquinoline)aluminium: Alq3), and made a functioning OLED device. However, they had not developed a technique for photo-lithographically patterning the emissive layer, and unless the emissive layer can also be patterned, only a monochrome device can be formed.
The present invention is directed to OLEDs that solve some of the problems in the prior art.